This invention relates to in situ recovery of shale oil, and more particularly, to techniques involving the excavation and explosive expansion of oil shale formation in preparation for forming an in situ oil shale retort.
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating in the absence of oxygen decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil."
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact.
The in situ recovery of liquid and gaseous products from oil shale deposits have been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; and 4,043,598 which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded for forming a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting." Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products, and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooled oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
It is desirable to form a fragmented mass having a generally uniformly distributed void fraction, i.e., a fragmented mass of generally uniform permeability, so that oxygen-supplying gas can flow relatively uniformly through the fragmented mass during retorting operations. Techniques used for excavating void spaces in a retorting site and for explosively expanding formation toward the voids can affect the uniformity of particle size and/or permeability of the fragmented mass. A fragmented mass having generally uniform permeability in horizontal planes across the fragmented mass avoids bypassing portions of the fragmented mass by retorting gas, which can occur if there is gas channeling through the fragmented mass owing to non-uniform permeability.
The creation of a fragmented mass of relatively uniform permeability distribution not only prevents the formation of channels which hinder total recovery of shale oil, but it also provides a relatively uniform pressure drop through the entire fragmented mass. In the preparation of a fragmented mass, it is important that formation be fragmented and displaced, rather than simply fractured, in order to form a fragmented mass of generally high permeability. It is also important that the retort contain a substantially uniformly fragmented mass of particles so uniform conversion of kerogen to liquid and gaseous products occurs during retorting. A wide distribution of particle size can adversely affect the efficiency of retorting because small particles can be completely retorted long before the cores of large particles are completely retorted.
It has been proposed that oil shale be prepared for in situ recovery by first undercutting a portion of the formation to remove from about 5% to about 25% of the total volume of the in situ retort being formed. The overlying formation is then expanded by detonating explosives placed in the formation to fill the void space created by the undercut.
The general art of blasting rock formations is discussed in The Blaster's Handbook, 15th Edition, published by E. I. DuPont de Nemours & Company, Wilmington, Delaware.
Another method for preparing oil shale formation for in situ recovery is described in U.S. Pat. Nos. 4,043,597, and 4,043,598, where two voids vertically spaced apart from each other are excavated in a subterranean formation. This leaves a zone of unfragmented formation between the voids. Vertical blasting holes are drilled in the intervening zone. Explosive is placed in the blasting holes and detonated to expand formation in the intervening zone toward both voids.